Thin-film acoustically-coupled transformer

ABSTRACT

One embodiment of the acoustically-coupled transformer includes a stacked bulk acoustic resonator (SBAR) having a stacked pair of film bulk acoustic resonators (FBARs) with an acoustic decoupler between them. Each FBAR has opposed planar electrodes with piezoelectric material between them. The transformer additionally has first terminals electrically connected to the electrodes of one FBAR and second terminals electrically connected to the electrodes of the other FBAR. Another embodiment includes first and second stacked bulk acoustic resonators (SBARs), each as described above, a first electrical circuit connecting one FBARs of the first SBAR to one FBAR of the second SBAR, and a second electrical circuit connecting the other FBAR of the first SBAR to the other FBAR of the second SBAR. The transformer provides impedance transformation, can linking single-ended circuitry with balanced circuitry or vice versa and electrically isolates primary and secondary. Some embodiments are additionally electrically balanced.

RELATED APPLICATION

This application is related to U.S. patent application Ser. No. ______of John D. Larson III entitled Stacked Bulk Acoustic Resonator Band-PassFilter with Controllable Pass Bandwidth (Agilent Docket No. 10030669),filed on the filing date of this application and incorporated into thisapplication by reference.

BACKGROUND OF THE INVENTION

Transformers are used in many types of electronic device to perform suchfunctions as transforming impedances, linking single-ended circuitrywith balanced circuitry or vice versa and providing electricalisolation. However, not all transformers have all of these properties.For example, an auto-transformer does not provide electrical isolation.

Transformers operating at audio and radio frequencies up to VHF arecommonly built as coupled primary and secondary windings around a highpermeability core. The core contains the magnetic flux and increases thecoupling between the windings. A transformer operable in this frequencyrange can also be realized using an optical-coupler. An opto-couplerused in this mode is referred to in the art as an opto-isolator.

In transformers based on coupled windings or opto-couplers, the inputelectrical signal is converted to a different form (i.e., a magneticflux or photons) that interacts with an appropriate transformingstructure (i.e., another winding or a light detector), and isre-constituted as an electrical signal at the output. For example, anopto-coupler converts an input electrical signal to photons using alight-emitting diode. The photons pass through an optical fiber or freespace that provides isolation. A photodiode illuminated by the photonsgenerates an output electrical signal from the photon stream. The outputelectrical signal is a replica of the input electrical signal.

At UHF and microwave frequencies, coil-based transformers becomeimpractical due to such factors as losses in the core, losses in thewindings, capacitance between the windings, and a difficulty to makethem small enough to prevent wavelength-related problems. Transformersfor such frequencies are based on quarter-wavelength transmission lines,e.g., Marchand type, series input/parallel output connected lines, etc.Transformers also exist that are based on micro-machined coupled coilssets and are small enough that wavelength effects are unimportant.However such transformers have issues with high insertion loss.

All the transformers just described for use at UHF and microwavefrequencies have dimensions that make them less desirable for use inmodern miniature, high-density applications such as cellular telephones.Such transformers also tend to be high in cost because they are notcapable of being manufactured by a batch process and because they areessentially an off-chip solution. Moreover, although such transformerstypically have a bandwidth that is acceptable for use in cellulartelephones, they typically have an insertion loss greater than 1 dB,which is too high.

Opto-couplers are not used at UHF and microwave frequencies due to thejunction capacitance of the input LED, non-linearities inherent in thephotodetector and insufficient isolation to give good common moderejection.

What is needed, therefore, is a transformer capable of providing one ormore of the following attributes at electrical frequencies in the rangefrom UHF to microwave: impedance transformation, coupling betweenbalanced and unbalanced circuits and electrical isolation. What is alsoneeded is such a transformer that has a low insertion loss, a bandwidthsufficient to accommodate the frequency range of cellular telephone RFsignals, for example, a size smaller than transformers currently used incellular telephones and a low manufacturing cost.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides an acoustically-coupledtransformer that comprises a stacked bulk acoustic resonator (SBAR) thathas a stacked pair of film bulk acoustic resonators (FBARs) and anacoustic decoupler between the FBARs. Each of the FBARs has opposedplanar electrodes and a layer of piezoelectric material between theelectrodes. The acoustically-coupled transformer additionally comprisesfirst terminals electrically connected to the electrodes of one of theFBARs and second terminals electrically connected to the electrodes ofthe other of the FBARs. The acoustically-coupled transformer has a 1:1impedance transformation ratio, is capable of linking single-endedcircuitry with balanced circuitry or vice versa and provides electricalisolation between primary and secondary.

In one embodiment, the acoustic decoupler includes a layer of acousticdecoupling material having an acoustic impedance less than that of theother materials of the FBARs. In another embodiment, the acousticdecoupler includes a Bragg structure.

In a second aspect, the invention provides an acoustically-coupledtransformer that has a first stacked bulk acoustic resonator (SBAR) anda second SBAR. Each SBAR has a stacked pair of film bulk acousticresonators (FBARs) and an acoustic decoupler between the FBARs. Each ofthe FBARs has opposed planar electrodes and a layer of piezoelectricmaterial between the electrodes. The acoustically-coupled transformeradditionally has a first electrical circuit connecting one of the FBARsof the first SBAR to one of the FBARs of the second SBAR and a secondelectrical circuit connecting the other of the FBARs of the first SBARto the other of the FBARs of the second SBAR. All embodiments of theacoustically-coupled transformer are capable of linking single-endedcircuitry with balanced circuitry or vice versa, and provides electricalisolation between primary and secondary.

Some embodiments of the acoustically-coupled transformer in accordancewith the invention are inherently electrically balanced and have ahigher common-mode rejection ratio than the above-described embodimenthaving the single SBAR. In such embodiments, the first electricalcircuit electrically connects one of the FBARs of the first SBAR eitherin anti-parallel or in series with one of the FBARs of the second SBAR,and the second electrical circuit electrically connects the other of theFBARs of the first SBAR either in anti-parallel or in series with theother of the FBARs of the second SBAR. An embodiment of theacoustically-coupled transformer in which the first electrical circuitconnects the respective FBARs in anti-parallel and the second electricalcircuit connects the respective FBARs in anti-parallel has a 1:1impedance transformation ratio between the first electrical circuit andthe second electrical circuit and vice versa. An embodiment in which thefirst electrical circuit connects the respective FBARs in series and thesecond electrical circuit connects the respective FBARs in series alsohas a 1:1 impedance transformation ratio between the first electricalcircuit and the second electrical circuit and vice versa. However, theimpedances are higher than the embodiment in which the FBARs areconnected in anti-parallel. An embodiment of the acoustically-coupledtransformer in which the first electrical circuit connects therespective FBARs in anti-parallel and the second electrical circuitconnects the respective FBARs in series has a 1:4 impedancetransformation ratio between the first electrical circuit and the secondelectrical circuit and a 4:1 impedance transformation ratio between thesecond electrical circuit and the first electrical circuit. Anembodiment of the acoustically-coupled transformer in which the firstelectrical circuit connects the respective FBARs in series and thesecond electrical circuit connects the respective FBARs in anti-parallelhas a 4:1 impedance transformation ratio between the first electricalcircuit and the second electrical circuit and a 1:4 impedancetransformation ratio between the second electrical circuit and the firstelectrical circuit.

Other embodiments of the acoustically-coupled transformer in accordancewith the invention are electrically unbalanced and can be used inapplications in which a high common-mode rejection ratio is lessimportant. In such embodiments, the first electrical circuitelectrically connects one of the FBARs of the first SBAR either inparallel or in anti-series with one of the FBARs of the second SBAR, andthe second electrical circuit electrically connects the other of theFBARs of the first SBAR either in parallel or in anti-series with theother of the FBARs of the second SBAR.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of an example of a first embodiment of athin-film acoustically-coupled transformer in accordance with theinvention

FIGS. 1B and 1C are cross-sectional views of the thin-filmacoustically-coupled transformer along section lines 1B-1B and 1C-1C,respectively, in FIG. 1A.

FIG. 1D is an enlarged cross-sectional view of part of theacoustically-coupled transformer shown in FIG. 1A along the section line1B-1B showing a first embodiment of the acoustic decoupler.

FIG. 1E is an enlarged cross-sectional view of part of theacoustically-coupled transformer shown in FIG. 1A along the section line1B-1B showing a second embodiment of the acoustic decoupler.

FIG. 2 a graph showing how the calculated frequency response ofembodiments of the thin-film acoustically-coupled transformer shown inFIGS. 1A-1C depends on the acoustic impedance of the acoustic decouplingmaterial.

FIG. 3A is a plan view of an example of a second embodiment of athin-film acoustically-coupled transformer in accordance with theinvention

FIGS. 3B and 3C are cross-sectional views of the thin-filmacoustically-coupled transformer along section lines 3B-3B and 3C-3C,respectively, in FIG. 1A.

FIGS. 4A through 4D are schematic drawings showing the electricalcircuits of electrically balanced embodiments of the thin-filmacoustically-coupled transformer shown in FIGS. 3A-3C.

FIGS. 4E through 4H are schematic drawings showing the electricalcircuits of electrically unbalanced embodiments of the thin-filmacoustically-coupled transformer shown in FIGS. 3A-3C.

FIGS. 5A-5J are plan views illustrating a process for making a thin-filmacoustically-coupled transformer in accordance with the invention.

FIGS. 5K-5S are cross-sectional views along the section lines 5K-5K,5L-5L, 5M-5M, 5N-5N, 5O-5O, 5P-5P, 5Q-5Q, 5R-5R, 5S-5S and 5T-5T inFIGS. 5A-5J, respectively.

DETAILED DESCRIPTION

FIGS. 1A, 1B and 1C show a plan view and two cross-sectional views,respectively, of a first embodiment 100 of a thin-filmacoustically-coupled transformer in accordance with the invention.Transformer 100 has a 1:1 impedance transformation ratio, is capable oflinking single-ended circuitry with balanced circuitry or vice versa andprovides electrical isolation between primary and secondary.

Transformer 100 is composed of a stacked bulk acoustic resonator (SBAR)106, first terminals 132 and 134 and second terminals 136 and 138. SBAR106 is composed of a stacked pair of film bulk acoustic resonators(FBARs) 110 and 120 and an acoustic decoupler 130 between them. In theexample shown, FBAR 120 is stacked atop FBAR 110. FBAR 110 is composedof opposed planar electrodes 112 and 114 and a layer of piezoelectricmaterial 116 between the electrodes. FBAR 120 is composed of opposedplanar electrodes 122 and 124 and a layer of piezoelectric material 126between the electrodes. Acoustic decoupler 130 is located betweenelectrode 114 of FBAR 110 and electrode 122 of FBAR 120. The acousticdecoupler controls the coupling of acoustic energy between FBARs 110 and120.

In the example shown, first terminals 132 and 134 are structured asbonding pads electrically connected by electrical traces 133 and 135,respectively, to electrodes 112 and 114, respectively, of FBAR 110. Alsoin the example shown, second terminals 136 and 138 are structured asbonding pads electrically connected by electrical traces 137 and 139,respectively, to electrodes 122 and 124, respectively, of FBAR 120. Inan embodiment, first terminals 132 and 134 constitute the primaryterminals and the second terminals 136 and 138 constitute the secondaryterminals of thin-film acoustically-coupled transformer 100. In analternative embodiment, first terminals 132 and 134 constitute thesecondary terminals and second terminals 136 and 138 constitute theprimary terminals of thin-film acoustically-coupled transformer 100.

In the example shown, SBAR 106 is suspended over a cavity 104 defined ina substrate 102. Suspending the SBAR over a cavity allows the FBARs ofthe SBAR to resonate mechanically. Other suspension schemes that allowthe FBARs to resonate mechanically are possible. For example, the SBARcan be located over a mismatched acoustic Bragg reflector (not shown)formed in or on substrate 102, as disclosed by Lakin in U.S. Pat. No.6,107,721, the disclosure of which is incorporated into this disclosureby reference.

FBARs are disclosed by Ruby et al. in U.S. Pat. No. 5,587,620 entitledTunable Thin Film Acoustic Resonators and Method of Making Same, nowassigned to the assignee of this disclosure and incorporated in thisdisclosure by reference: Ruby's disclosure also discloses a stacked filmbulk acoustic resonator (SBAR) composed of two layers of piezoelectricmaterial interleaved with three planar electrodes. Ruby's SBAR can beregarded as being composed of a stacked pair of FBARs in which oneelectrode is common to both FBARs, and will be referred to as acommon-electrode SBAR. The common electrode renders the common-electrodeSBAR incapable of linking balanced to unbalanced circuits and vice versaand of providing electrical isolation between primary and secondary.Moreover, the common electrode SBAR exhibits an extremely narrow passbandwidth that makes it unsuitable for use in most applications. Thenarrow pass bandwidth is the result of the common electrode, which overcouples acoustic energy between the FBARs.

As noted above, in transformer 100 in accordance with the invention,acoustic decoupler 130 controls the coupling of acoustic energy betweenstacked FBARs 110 and 120 and additionally electrically isolates FBAR110 from FBAR 120. The electrical isolation provided by acousticdecoupler 130 enables transformer 100 to link balanced to unbalancedcircuits and vice versa and provides electrical isolation betweenprimary and secondary. The acoustic coupling provided by acousticdecoupler 130 is substantially less than the acoustic coupling betweenthe FBARs in the common electrode SBAR referred to above. As a result,FBARs 110 and 120 are not over coupled, and transformer 100 has arelatively flat response in the pass band, as will be described belowwith reference to FIG. 2.

The embodiment of the acoustic decoupler 130 shown in FIGS. 1A-1C is afirst embodiment composed of layer 131 of acoustic decoupling materiallocated between the electrodes 114 and 122 of FBARs 110 and 120,respectively. FIG. 1D is an enlarged view showing this first embodimentof the acoustic decoupler in more detail. Important properties of theacoustic decoupling material of layer 131 that constitutes acousticdecoupler 130 are an acoustic impedance less than that of the materialsof FBARs 110, 120, a high electrical resistivity, a low dielectricpermittivity and a nominal thickness that is an odd integral multiple ofone quarter of the wavelength in the acoustic decoupling material of anacoustic wave having a frequency equal to the center frequency of thepass band of acoustically-coupled transformer 100.

The acoustic decoupling material of acoustic decoupler 130 has anacoustic impedance less that of the materials of FBARs 110 and 120 andsubstantially greater than that of air. The acoustic impedance of amaterial is the ratio of stress to particle velocity in the material andis measured in Rayleighs, abbreviated as rayl. The materials of theFBARs are typically aluminum nitride (AlN) as the material ofpiezoelectric layers 116, 126 and molybdenum (Mo) as the material ofelectrodes 112, 114, 122 and 124. The acoustic impedances of thematerials of the FBARs are typically greater than 30 Mrayl (35 Mrayl forAlN and 63 Mrayl for Mo) and the acoustic impedance of air is about 1krayl. In embodiments of transformer 100 in which the materials of FBARs110, 120 are as stated above, materials with an acoustic impedance inthe range from about 2 Mrayl to about 16 Mrayl work well as the acousticcoupling material of acoustic decoupler 130.

FIG. 2 is a graph showing how the calculated frequency response ofthin-film acoustically-coupled transformer 100 depends on the acousticimpedance of the acoustic decoupling material of layer 131 thatconstitutes the first embodiment of acoustic decoupler 130. Theembodiment illustrated has a center frequency of about 1,900 MHz.Calculated frequency responses for embodiments in which the acousticdecoupling material of the acoustic decoupler has acoustic impedances ofabout 4 Mrayl (polyimide-curve 140), 8 Mrayl (curve 142) and 16 Mrayl(curve 144) are shown. It can be seen that the bandwidth of transformer100 increases with increasing acoustic impedance of the acousticdecoupling material. In the embodiment in which the acoustic impedanceis 16 Mrayl, the resonances of the FBARs are over coupled, which causesthe characteristic double peak in the pass band response.

The embodiment of acoustic decoupler 130 shown in FIGS. 1B, 1C and 1D iscomposed of layer 131 of acoustic decoupling material with a nominalthickness equal to one quarter of the wavelength in the acousticdecoupling material of an acoustic wave having a frequency equal to thecenter frequency of the transformer's pass band, i.e., t≈λ_(n)/4, wheret is the thickness of the layer 131 of acoustic decoupling material thatconstitutes acoustic decoupler 130 and λ_(n) is the wavelength in theacoustic decoupling material of an acoustic wave having a frequencyequal to the center frequency of the pass band of transformer 100. Athickness of layer 131 within approximately ±10% of the nominalthickness can alternatively be used. A thickness outside this range canalternatively be used with some degradation in performance. However, thethickness of layer 131 should differ significantly from 0λ_(n) at oneextreme and λ_(n)/2 at the other extreme.

More generally, the first embodiment of acoustic decoupler 130 shown inFIG. 1D is composed of layer 131 of acoustic decoupling material with anominal thickness equal to an odd integral multiple of one quarter ofthe wavelength in the acoustic decoupling material of an acoustic wavehaving a frequency equal to the center frequency of the pass band oftransformer 100, i.e., t≈(2m+1)λ_(n)/4, where t and λ_(n) are as definedabove and m is an integer equal to or greater than zero. In this case, athickness of layer 131 that differs from the nominal thickness byapproximately ±10% of λ_(n)/4 can alternatively be used. A thicknesstolerance outside this range can be used with some degradation inperformance, but the thickness of layer 131 should differ significantlyfrom an integral multiple of λ_(n)/2.

Many plastic materials have acoustic impedances in the range statedabove and can be applied in layers of uniform thickness in the thicknessranges stated above. Such plastic materials are therefore potentiallysuitable for use as the acoustic decoupling material of layer 131 ofacoustic decoupler 130. However, the acoustic decoupling material mustalso be capable of withstanding the temperatures of the fabricationoperations performed after layer 131 of acoustic decoupling material hasbeen deposited on electrode 114 to form acoustic decoupler 130. As willbe described in more detail below, in practical embodiments of thin-filmacoustically-coupled transformer 100, electrodes 122 and 124 andpiezoelectric layer 126 are deposited by sputtering after layer 131 hasbeen deposited. Temperatures as high as 300° C. are reached during thesedeposition processes. Thus, a plastic that remains stable at suchtemperatures is used as the acoustic decoupling material.

Plastic materials typically have a very high acoustical attenuation perunit length compared with the other materials of FBARs 110 and 120.However, since the above-described embodiment of acoustic decoupler 130is composed of layer 131 of plastic acoustic decoupling materialtypically less than 1 μm thick, the acoustic attenuation introduced bylayer 131 is typically negligible.

In one embodiment, a polyimide is used as the acoustic decouplingmaterial of layer 131. Polyimide is sold under the trademark Kapton® byE. I. du Pont de Nemours and Company. In such embodiment, acousticdecoupler 130 is composed of layer 131 of polyimide applied to electrode114 by spin coating. Polyimide has an acoustic impedance of about 4Mrayl. In another embodiment, a poly(para-xylylene) is used as theacoustic decoupling material of layer 131. In such embodiment, acousticdecoupler 130 is composed of layer 131 of poly(para-xylylene) applied toelectrode 114 by vacuum deposition. Poly(para-xylylene) is also known inthe art as parylene. The dimer precursor di-para-xylylene from whichparylene is made and equipment for performing vacuum deposition oflayers of parylene are available from many suppliers. Parylene has anacoustic impedance of about 2.8 Mrayl.

In an alternative embodiment, the acoustic decoupling material of layer131 constituting acoustic decoupler 130 has an acoustic impedancesubstantially greater than the materials of FBARs 110 and 120. Nomaterials having this property are known at this time, but suchmaterials may become available in future, or lower acoustic impedanceFBAR materials may become available in future. The thickness of layer131 of such high acoustic impedance acoustic decoupling material is asdescribed above.

FIG. 1E is an enlarged view of part of thin-film acoustically-coupledtransformer 100 showing a second embodiment of acoustic decoupler 130that incorporates a Bragg structure 161. Bragg structure 161 is composedof a low acoustic impedance Bragg element 163 sandwiched between highacoustic impedance Bragg elements 165 and 167. Low acoustic impedanceBragg element 163 is a layer of a low acoustic impedance materialwhereas high acoustic impedance Bragg elements 165 and 167 are each alayer of high acoustic impedance material. The acoustic impedances ofthe Bragg elements are characterized as “low” and “high” with respect toone another and additionally with respect to the acoustic impedance ofthe piezoelectric material of layers 116 and 126. At least one of theBragg elements additionally has a high electrical resistivity and a lowdielectric permittivity to provide electrical isolation between inputand output of transformer 100.

Each of the layers constituting Bragg elements 161, 163 and 165 has anominal thickness equal to an odd integral multiple of one quarter ofthe wavelength in the material the layer of an acoustic wave having afrequency equal to the center frequency of transformer 100. Layers thatdiffer from the nominal thickness by approximately ±10% of one quarterof the wavelength can alternatively be used. A thickness toleranceoutside this range can be used with some degradation in performance, butthe thickness of the layers should differ significantly from an integralmultiple of one-half of the wavelength.

In an embodiment, low acoustic impedance Bragg element 163 is a layer ofsilicon dioxide (SiO₂), which has an acoustic impedance of about 13Mrayl, and each of the high acoustic impedance Bragg elements 165 and167 is a layer of the same material as electrodes 114 and 122,respectively, i.e., molybdenum, which has an acoustic impedance of about63 Mrayl. Using the same material for high acoustic impedance Braggelements 165 and 167 and electrodes 114 and 122, respectively, of FBARs110 and 120, respectively, allows high acoustic impedance Bragg elements165 and 167 additionally to serve as electrodes 114 and 122,respectively.

In an example, high acoustic impedance Bragg elements 165 and 167 have anominal thickness equal to one quarter of the wavelength in molybdenumof an acoustic wave having a frequency equal to the center frequency ofthe pass band of transformer 100, and low acoustic impedance Braggelement 163 had a nominal thickness equal to three quarters of thewavelength in SiO₂ of an acoustic wave having a frequency equal to thecenter frequency of the pass band of the transformer. Using athree-quarter wavelength-thick layer of SiO₂ instead of a one-quarterwavelength thick layer of SiO₂ as low acoustic impedance Bragg element163 reduces the capacitance between FBARs 110 and 120.

In embodiments in which the acoustic impedance difference between highacoustic impedance Bragg elements 165 and 167 and low acoustic impedanceBragg element 163 is relatively low, Bragg structure 161 may be composedof more than one (e.g., n) low acoustic impedance Bragg elementinterleaved with a corresponding number (i.e., n+1) of high acousticimpedance Bragg elements. Only one of the Bragg elements need beinsulating. For example, the Bragg structure may be composed of two lowacoustic impedance Bragg element interleaved with three high acousticimpedance Bragg elements.

Wafer-scale fabrication is used to fabricate thin-filmacoustically-coupled transformers similar to thin-filmacoustically-coupled transformer 100 thousands at a time. Wafer-scalefabrication makes each thin-film acoustically-coupled transformerinexpensive to fabricate. Thin-film acoustically-coupled transformer 100can be made using a fabrication method similar to that to be describedbelow with reference to FIGS. 5A-5T. Accordingly, a method offabricating thin-film acoustically-coupled transformer 100 will not beseparately described.

Referring again to FIGS. 1A-1C, to use thin-film acoustically-coupledtransformer 100, electrical connections are made to first terminals 132and 134 electrically connected to electrodes 112 and 114, respectively,as shown in FIGS. 1A and 1B and electrical connections are additionallymade to second terminals 136 and 138 electrically connected toelectrodes 122 and 124, respectively, as shown in FIGS. 1A and 1C. Theelectrical connections to first terminals 132 and 134 provide electricalconnections to the primary of thin-film acoustically-coupled transformer100 and the electrical connections to second terminals 136 and 138provide electrical connections to the secondary of thin-filmacoustically-coupled transformer 100. In an alternative embodiment, theelectrical connections to second terminals 136 and 138 provideelectrical connections to the primary of thin-film acoustically-coupledtransformer 100 and the electrical connections to first terminals 132and 134 provide electrical connections to the secondary of thin-filmacoustically-coupled transformer 100.

In operation of thin-film acoustically-coupled transformer 100, an inputelectrical signal applied to first terminals 132 and 134, whichconstitute the primary terminals of thin-film acoustically-coupledtransformer 100, establishes a voltage difference between electrodes 112and 114 of FBAR 110. The voltage difference between electrodes 112 and114 mechanically deforms FBAR 110 at the frequency of the inputelectrical signal. Depending on the frequency of the input electricalsignal, acoustic decoupler 130 couples all or part of the acousticenergy resulting from the mechanical deformation of FBAR 110 to FBAR120. The acoustic energy received from FBAR 110 mechanically deformsFBAR 120 at the frequency of the input electrical signal. The mechanicaldeformation of FBAR 120 generates a voltage difference betweenelectrodes 122 and 124 at the frequency of the input electrical signal.The voltage difference is output at second terminals 136 and 138, whichconstitute the secondary terminals of transformer 100, as an outputelectrical signal. Piezoelectricity is a linear effect, so the amplitudeand phase of the input electrical signal applied to the first terminalsis preserved in the output electrical signal output at the secondterminals.

An embodiment of thin-film acoustically-coupled transformer 100 in whichsecond terminals 136 and 138 constitute the primary terminals and firstterminals 132 and 134 constitute the secondary terminals operatessimilarly, except acoustic energy propagates through acoustic decoupler130 from FBAR 120 to FBAR 110.

As noted above, thin-film acoustically-coupled transformer 100 providesa 1:1 impedance transformation ratio, is capable of linking single-endedcircuitry with balanced circuitry or vice versa and provides electricalisolation between primary and secondary. However, the capacitancebetween electrode 112 and substrate 102 differs from that betweenelectrode 114 and the substrate. As a result, thin-filmacoustically-coupled transformer 100 is not perfectly balancedelectrically and can have an insufficient common-mode rejection ratio(CMRR) for certain applications.

FIGS. 3A-3C show a plan view and two cross-sectional views,respectively, of a second embodiment 200 of a thin-filmacoustically-coupled transformer in accordance with the invention.Acoustically-coupled transformer 200 is capable of linking single-endedcircuitry with balanced circuitry or vice versa, provides electricalisolation between primary and secondary. Some embodiments of transformer200 are electrically balanced, and therefore have a high common-moderejection ratio: other embodiments are electrically unbalanced and havea lower common-mode rejection ratio. Acoustically-coupled transformer200 has an impedance transformation ratio of 1:1, 1:4 or 4:1 dependingon the configurations of the electrical circuits that form part of thetransformer.

Acoustically-coupled transformer 200 is composed of two stacked bulkacoustic resonators (SBARS) 206 and 208. Each SBAR is composed of astacked pair of film bulk acoustic resonators (FBARs) and an acousticdecoupler between the FBARs. Transformer 200 is additionally composed ofan electrical circuit that connects one of the FBARs of SBAR 206 to oneof the FBARs of SBAR 208, and an electrical circuit that connects theother of the FBARs of SBAR 206 to the other of the FBARs of SBAR 208.

SBAR 206 is composed of a stacked pair of FBARs 210 and 220 and anacoustic decoupler 230 between them. SBAR 208 is composed of a stackedpair of FBARs 250 and 260 and an acoustic decoupler 270 between them. Inthe example shown, FBAR 220 is stacked atop FBAR 210 and FBAR 260 isstacked atop FBAR 250. FBAR 210 is composed of opposed planar electrodes212 and 214 and a layer of piezoelectric material 216 between theelectrodes. FBAR 220 is composed of opposed planar electrodes 222 and224 and a layer of piezoelectric material 226 between the electrodes.FBAR 250 is composed of opposed planar electrodes 252 and 254 and alayer of piezoelectric material 256 between the electrodes. FBAR 260 iscomposed of opposed planar electrodes 262 and 264 and a layer ofpiezoelectric material 266 between the electrodes.

As noted above, an electrical circuit connects one of the FBARs of SBAR206 to one of the FBARs of SBAR 208, and an electrical circuit connectsthe other of the FBARs of SBAR 206 to the other of the FBARs of SBAR206. Each electrical circuit electrically connects the respective FBARsin any one of a parallel, a series, an anti-parallel and an anti-seriesconfiguration. Of the sixteen possible combinations of the parallel,series, anti-parallel and anti-series electrical circuit configurations,only eight produce a working transformer. The combination of electricalcircuit configurations connecting the FBARs determines whether thetransformer is electrically balanced (high common-mode rejection ratio)or electrically unbalanced, and determines the impedance transformationratio of the transformer, i.e., 1:1, 1:4 or 4:1. The possiblecombinations of electrical circuit configurations are summarized inTable 1 below: TABLE 1 Parallel Series Anti-par. Anti-series Parallel U1:1 X X U 1:4 Series X B 1:1 B 4:1 X Anti-par. X B 1:4 B 1:1 XAnti-series U 4:1 X X U 1:1

In Table 1, the row captions indicate the configuration of one of theelectrical circuits, e.g., electrical circuit 245 described below withreference to FIG. 4C, the column captions indicate the configuration ofthe other of the electrical circuits, e.g., electrical circuit 246described with reference to FIG. 4C, B denotes that the transformer iselectrically balanced, U denotes that the transformer is unbalanced, andX denotes a non-functioning transformer. The impedance transformationratio shown is the impedance transformation from electrical terminalsconnected to the electrical circuit indicated by the row caption toelectrical terminals connected to the electrical circuit indicated bythe column caption.

The electrical circuits shown in Table 1 are subject to the constraintthat an electrical circuit may only connect the electrodes of FBARs atthe same level as one another in SBARs 206 and 208, i.e., one of theelectrical circuits may only connect the electrodes of FBARs 210 and 250and the other of the electrical circuits may only connect the electrodesof FBARs 220 and 260. Table 1 additionally assumes that the c-axes ofpiezoelectric layers 216, 226, 256 and 266 are all oriented in the samedirection. More electrical circuits are possible in embodiments notsubject to the constraint, e.g., in embodiments in which an electricalcircuit is allowed to connect the electrodes of FBARs 210 and 260 andthe electrodes of FBARs 220 and 250, and/or the assumption.

Before the electrical circuits interconnecting the FBARs are describedin detail, the terms anti-parallel, parallel, anti-series and series asapplied to the electrical circuits connecting the electrodes of FBARs ofdifferent SBARs will be defined. An FBAR is a polarity-dependent device.A voltage of a given polarity applied between the electrodes of the FBARwill cause the FBAR to contract mechanically while the same voltage ofthe opposite polarity will cause the FBAR to expand mechanically by thesame amount. Similarly, a mechanical stress applied to the FBAR thatcauses the FBAR to contract mechanically will generate a voltage of thegiven polarity between the electrodes of the FBAR whereas a mechanicalstress that causes the FBAR to expand mechanically will generate avoltage of the opposite polarity between the electrodes of the FBAR.

Referring to FIGS. 4A-4D, in acoustically-coupled transformer 200, theelectrodes of the FBARs that an electrical circuit connects in parallelare at the same level in the respective SBARs. A signal applied to theFBARs connected in parallel produces signals of the same phase acrossthe FBARs. The FBARs therefore expand and contract in phase, andgenerate acoustic energy in phase. On the other hand, electrodes of theFBARs that an electrical circuit connects in anti-parallel are atdifferent levels in the respective SBARs. A signal applied to FBARsconnected in anti-parallel produces signals of the opposite phasesacross the FBARs. The FBARs therefore expand and contract in antiphase,and generate acoustic energy in antiphase.

The electrodes of the FBARs that an electrical circuit connects inseries are at the same level in the respective SBARs. A signal appliedto the FBARs connected in series produces signals of opposite phasesacross the FBARs. The FBARs expand and contract in antiphase, andgenerate acoustic energy in antiphase. On the other hand, the electrodesof the FBARs that an electrical circuit connects in anti-series are atdifferent levels in the respective SBARs. A signal applied to the FBARsconnected in anti-series produces signals of the same phase across theFBARs. The FBARs expand and contract in phase and generate acousticenergy in phase.

FBARs receiving acoustic energy that causes them to expand and contractin phase generate signals in phase. Connecting FBARs that generatesignals in phase in parallel produces a signal level equal to thatacross the individual FBARs and an impedance of one-half thecharacteristic impedance of the individual FBARs. Connecting such FBARsin anti-series produces a signal level of twice that across theindividual FBARs and an impedance of twice the characteristic impedanceof the individual FBARs. However, connecting FBARs that generate signalsin phase in anti-parallel or in series causes the signals to cancel.FBARs receiving acoustic energy that causes them to expand and contractin antiphase generate signals in antiphase. Connecting FBARs thatgenerate signals in antiphase in antiparallel produces a signal equal inlevel to that across the individual FBARs and an impedance of one-halfthe characteristic impedance of the individual FBARs. Connecting suchFBARs in series produces a signal of twice the level of that across theindividual FBARs and an impedance of twice the characteristic impedanceof the individual FBARs. However, connecting FBARs that generate signalsin antiphase in parallel or in antiseries causes the signals to cancel.The transformers indicated in Table 1 as being non-functional aretransformers in which the FBARs that receive acoustic energy generatesignals that cancel.

FIGS. 4A and 4B schematically illustrate two configurations ofelectrical circuits that connect the FBARs 210 and 220 of SBAR 206 andthe FBARs 250 and 260 of SBAR 208 in anti-parallel or in series,respectively, to form respective electrically-balanced embodiments of anacoustically-coupled transformer having a 1:1 impedance transformationratio.

FIG. 4A shows an electrical circuit 241 electrically connecting one ofthe FBARs of SBAR 206 in anti-parallel with one of the FBARs of SBAR 208and to first terminals F and an electrical circuit 242 electricallyconnecting the other of the FBARs of SBAR 206 in anti-parallel with theother of the FBARs of SBAR 208 and to second terminals S. In the exampleshown, the electrical circuit 241 electrically connects FBAR 220 of SBAR206 in anti-parallel with FBAR 260 of SBAR 208 and to first terminals F,and electrical circuit 242 electrically connects FBAR 210 of SBAR 206 inanti-parallel with FBAR 250 of SBAR 208 and to second terminals S.

Specifically, electrical circuit 241 electrically connects electrode 222of FBAR 220 to electrode 264 of FBAR 260 and to one of the firstterminals F and additionally electrically connects electrode 224 of FBAR220 to electrode 262 of FBAR 260 and to the other of the first terminalsF. Electrical circuit 242 electrically connects electrode 214 of FBAR210 to electrode 252 of FBAR 250 and to one of the second terminals Sand additionally electrically connects electrode 212 of FBAR 210 toelectrode 254 of FBAR 250 and to the other of the second terminals S.

Electrical circuit 241 electrically connects FBARs 220 and 260 inanti-parallel so that an input electrical signal applied to the firstterminals F is applied equally but in antiphase to FBARs 220 and 260.Electrical circuit 241 electrically connects FBARs 220 and 260 inanti-parallel in the sense that an electrical signal applied to firstterminals F that causes FBAR 220 to contract mechanically additionallycauses FBAR 260 to expand mechanically by the same amount, and viceversa. The acoustic energy generated by FBAR 260 is therefore inantiphase with the acoustic energy generated by FBAR 220. Consequently,the acoustic energy received by FBAR 250 from FBAR 260 is in antiphasewith the acoustic energy received by FBAR 210 from FBAR 220, and thesignal between electrodes 214 and 212 is in antiphase with the signalbetween electrodes 254 and 252. Electrical circuit 242 connects FBARs210 and 250 in anti-parallel, so that the signal output to the secondterminals S is in phase with the signal between electrodes 214 and 212and also with the signal between electrodes 254 and 252. As a result,the signal between second terminals S is the same as the signal acrosseither of FBARs 210 and 250.

Substantially the same capacitance exists between each of the firstterminals F and substrate 202. Each first terminal has connected to itone electrode closer to the substrate and one electrode further from thesubstrate. In the example shown, one first terminal has electrode 222closer to the substrate and electrode 264 further from the substrateconnected to it and the other first terminal has electrode 262 closer tothe substrate and electrode 224 further from the substrate connected toit. Moreover, substantially the same capacitance exists between each ofthe second terminals S and substrate 202. Each second terminal hasconnected to it one electrode closer to the substrate and one electrodefurther from the substrate. In the example shown, one second terminalhas electrode 212 closer to the substrate and electrode 254 further fromthe substrate connected to it and the other second terminal haselectrode 252 closer to the substrate and electrode 214 further from thesubstrate connected to it. Thus, the embodiment of thin-filmacoustically-coupled transformer 200 shown in FIG. 4A is electricallybalanced and, as a result, has a common-mode rejection ratiosufficiently high for many more applications than the thin-filmacoustically-coupled transformer 100 described above with reference toFIGS. 1A-1C.

The embodiment of thin-film acoustically-coupled transformer 200 shownin FIG. 4A has a 1:1 impedance transformation ratio. First terminals Fmay serve as the primary terminals or the secondary terminals of thetransformer and second terminals P may serve as the secondary terminalsor the primary terminals, respectively, of the transformer. An inputelectrical signal applied to the primary terminals is output atsubstantially the same level at the secondary terminals. In a typicalembodiment in which all of the FBARs 210, 220, 250 and 260 have asimilar characteristic impedance, the impedance seen at the primaryterminals and at the secondary terminals is that of two FBARs inparallel, i.e., one half of the typical characteristic impedance of asingle FBAR. Thus, the embodiment of thin-film acoustically-coupledtransformer 200 shown in FIG. 4A is suitable for use in relatively lowcharacteristic impedance applications.

FIG. 4B schematically shows an electrical circuit 243 electricallyconnecting one of the FBARs of SBAR 206 and one of the FBARs of SBAR 208in series between first terminals F and an electrical circuit 244electrically connecting the other of the FBARs of SBAR 206 and the otherof the FBARs of SBAR 208 in series between second terminals S. In theexample shown in FIG. 4B, electrical circuit 243 electrically connectsFBAR 220 of SBAR 206 and FBAR 260 of SBAR 208 in series between firstterminals F, and electrical circuit 244 electrically connects FBAR 210of SBAR 206 and FBAR 250 of SBAR 208 in series between second terminalsS.

Specifically, electrical circuit 243 electrically connects electrode 222of FBAR 220 to electrode 262 of FBAR 260 and additionally electricallyconnects electrode 224 of FBAR 220 to one of the first terminals F andelectrically connects electrode 264 of FBAR 260 to the other of thefirst terminals F. In a variation, electrical circuit 243 electricallyconnects electrode 224 of FBAR 220 to electrode 264 of FBAR 260 andadditionally electrically connects electrode 222 of FBAR 220 andelectrode 262 of FBAR 260 to first terminals F. Electrical circuit 244electrically connects electrode 212 of FBAR 210 to electrode 252 of FBAR250 and additionally electrically connects electrode 214 of FBAR 210 toone of the second terminals S and additionally electrically connectselectrode 254 of FBAR 250 to the other of the second terminals S. In avariation, electrical circuit 244 electrically connects electrode 214 ofFBAR 210 to electrode 254 of FBAR 250 and additionally electricallyconnects electrode 212 of FBAR 210 and electrode 252 of FBAR 250 tosecond terminals S.

Electrical circuit 243 electrically connecting FBARs 220 and 260 inseries divides an input electrical signal applied to the first terminalsF approximately equally between FBARs 220 and 260. FBARs 220 and 260 areconnected in series in the sense that an electrical signal applied tofirst terminals F that causes FBAR 220 to contract mechanically causesFBAR 260 to expand mechanically by the same amount, and vice versa. Theacoustic energy generated by FBAR 260 is therefore in antiphase with theacoustic energy generated by FBAR 220. The acoustic energy received byFBAR 250 from FBAR 260 is in antiphase with the acoustic energy receivedby FBAR 210 from FBAR 220 and the signal on electrode 254 is inantiphase with the signal on electrode 214. Electrical circuit 244electrically connects FBARs 210 and 250 in series so that the signal atsecond terminals S is twice the signal across either of FBARs 210 and250.

Substantially the same capacitance exists between each of the firstterminals F and substrate 202. Electrodes 224 and 264 connected to thefirst terminals are at the same distance from the substrate. Moreover,substantially the same capacitance exists between each of the secondterminals S and substrate 202. Electrodes 214 and 254 connected to thesecond terminals are at the same distance from the substrate. Thus, theembodiment of thin-film acoustically-coupled transformer 200 shown inFIG. 4B is electrically balanced and, as a result, has a common-moderejection ratio sufficiently high for many more applications than thethin-film acoustically-coupled transformer 100 described above withreference to FIGS. 1A-1C.

The embodiment of thin-film acoustically-coupled transformer 200 shownin FIG. 4B has a 1:1 impedance transformation ratio. First terminals Fmay serve as the primary terminals or the secondary terminals of thetransformer and second terminals P may serve as the secondary terminalsor the primary terminals, respectively, of the transformer. An inputelectrical signal applied to the primary terminals is output atsubstantially the same level at the secondary terminals. In a typicalembodiment in which all of the FBARs 210, 220, 250 and 260 have asimilar characteristic impedance, the impedance seen at the primaryterminals and at the secondary terminals is that of two FBARs in series,i.e., twice the typical characteristic impedance of a single FBAR. Thus,the embodiment of thin-film acoustically-coupled transformer 200 shownin FIG. 4B is suitable for use in higher characteristic impedanceapplications than that shown in FIG. 4A.

FIGS. 4C and 4D schematically illustrate two configurations ofelectrical circuits that connect the FBARs 210 and 220 of SBAR 206 andthe FBARs 250 and 260 of SBAR 208 in anti-parallel and in series to formrespective embodiments of an acoustically-coupled transformer having a1:4 or 4:1 impedance transformation ratio. FIG. 4C shows an electricalcircuit 245 electrically connecting one of the FBARs of SBAR 206 inanti-parallel with one of the FBARs of SBAR 208 and to first terminals Fand an electrical circuit 246 electrically connecting the other of theFBARs of SBAR 206 and the other of the FBARs of SBAR 208 in seriesbetween second terminals S. In the example shown, the electrical circuit245 electrically connects FBAR 220 of SBAR 206 in anti-parallel withFBAR 260 of SBAR 208 and to first terminals P, and electrical circuit246 electrically connects FBAR 210 of SBAR 206 and FBAR 250 of SBAR 208in series between second terminals S.

Specifically, electrical circuit 245 electrically connects electrode 222of FBAR 220 to electrode 264 of FBAR 260 and to one of the firstterminals F, and additionally electrically connects electrode 224 ofFBAR 220 to electrode 262 of FBAR 260 and to the other of the firstterminals F. Electrical circuit 246 electrically connects electrode 214of FBAR 210 to electrode 254 of FBAR 250 and additionally electricallyconnects electrode 212 of FBAR 210 to one of the second terminals S andelectrode 252 of FBAR 250 to the other of the second terminals S. In avariation, electrical circuit 246 electrically connects electrode 212 ofFBAR 210 to electrode 252 of FBAR 250 and additionally electricallyconnects electrode 214 of FBAR 210 and electrode 254 of FBAR 250 tosecond terminals S.

Electrical circuit 245 electrically connects FBARs 220 and 260 inanti-parallel so that an input electrical signal applied to the firstterminals F is applied equally but in antiphase to FBARs 220 and 260.Electrical circuit 245 electrically connects FBARs 220 and 260 inanti-parallel in the sense that an electrical signal applied to firstterminals F that causes FBAR 220 to contract mechanically additionallycauses FBAR 260 to expand mechanically by the same amount, and viceversa. The acoustic energy generated by FBAR 260 is therefore inantiphase with the acoustic energy generated by FBAR 220. Consequently,the acoustic energy received by FBAR 250 from FBAR 260 is in antiphasewith the acoustic energy received by FBAR 210 from FBAR 220, and thesignal on electrode 252 is in antiphase with the signal on electrode212. Electrical circuit 246 connects FBARs 210 and 250 in series so thatthe voltage difference between second terminals S is twice the voltageacross either of FBARs 210 and 250.

Substantially the same capacitance exists between each of the firstterminals F and substrate 202. Each first terminal has connected to itone electrode closer to the substrate and one electrode further from thesubstrate. In the example shown, one first terminal has electrode 222closer to the substrate and electrode 264 further from the substrateconnected to it and the other first terminal has electrode 262 closer tothe substrate and electrode 224 further from the substrate connected toit. Moreover, substantially the same capacitance exists between each ofthe second terminals S and substrate 202. Electrodes 212 and 252connected to the second terminals are at the same distance from thesubstrate. Thus, the embodiment of thin-film acoustically-coupledtransformer 200 shown in FIG. 4C is electrically balanced and, as aresult, has a common-mode rejection ratio sufficiently high for manymore applications than the thin-film acoustically-coupled transformer100 described above with reference to FIGS. 1A-1C.

The embodiment of thin-film acoustically-coupled transformer 200 shownin FIG. 4C is a step-up transformer when first terminals F serve asprimary terminals and second terminals S serve as secondary terminals. Asignal applied to the primary terminals is output at twice the level atthe secondary terminals. Also, in a typical embodiment in which all ofthe FBARs 210, 220, 250 and 260 have a similar characteristic impedance,the impedance seen at the primary terminals is that of two FBARs inparallel, i.e., one half of the typical characteristic impedance of asingle FBAR, whereas the impedance seen at the secondary terminals isthat of two FBARs in series, i.e., twice the typical characteristicimpedance of a single FBAR. Thus, the embodiment of thin-filmacoustically-coupled transformer 200 illustrated in FIG. 4C has a 1:4primary-to-secondary impedance ratio.

The embodiment of thin-film acoustically-coupled transformer 200 shownin FIG. 4C is a step-down transformer when first terminals F serve assecondary terminals and second terminals S serve as primary terminals.In this case, the signal output at the secondary terminals is one-halfthe level of the input electrical signal applied to the primaryterminals, and the primary-to-secondary impedance ratio is 4:1.

FIG. 4D schematically shows an electrical circuit 247 electricallyconnecting FBAR 220 of SBAR 206 and FBAR 260 of SBAR 208 in seriesbetween first terminals F, and an electrical circuit 248 electricallyconnecting FBAR 210 of SBAR 206 and FBAR 250 of SBAR 208 inanti-parallel and to second terminals S.

Specifically, electrical circuit 247 electrically connects electrode 222of FBAR 220 to electrode 262 of FBAR 260 and additionally electricallyconnects electrode 224 of FBAR 220 and electrode 264 of FBAR 260 tofirst terminals F. Electrical circuit 248 electrically connectselectrode 212 of FBAR 210 to electrode 254 of FBAR 250 and to one of thesecond terminals S, and additionally electrically connects electrode 214of FBAR 210 to electrode 252 of FBAR 250 and to the other of the secondterminals S. In a variation, electrical circuit 247 electricallyconnects electrode 224 of FBAR 220 to electrode 264 of FBAR 260 andadditionally electrically connects electrode 222 of FBAR 220 andelectrode 262 of FBAR 260 to first terminals F.

Electrical circuit 247 electrically connecting FBARs 220 and 260 inseries divides an input electrical signal applied to the first terminalsF approximately equally between FBARs 220 and 260. FBARs 220 and 260 areconnected in series in the sense that an electrical signal applied tofirst terminals F that causes FBAR 220 to contract mechanically causesFBAR 260 to expand mechanically by the same amount, and vice versa. Theacoustic energy generated by FBAR 260 is therefore in antiphase with theacoustic energy generated by FBAR 220. The acoustic energy received byFBAR 250 from FBAR 260 is in antiphase with the acoustic energy receivedby FBAR 210 from FBAR 220 and the voltage between electrodes 252 and 254is in antiphase with the voltage between electrodes 212 and 214.Electrical circuit 248 electrically connects FBARs 210 and 250 inanti-parallel, so that the signal output at the second terminals S is inphase with the signal across electrodes 214 and 212 and also with thesignal across electrodes 254 and 252. As a result, the signal at secondterminals S is equal in level to the signal across either of FBARs 210and 250, and is equal to one-half the level of the input electricalsignal applied to first terminals F.

Substantially the same capacitance exists between each of the firstterminals F and substrate 202. Electrodes 224 and 264 connected to thefirst terminals are the at same distance from the substrate. Moreover,substantially the same capacitance exists between each of the secondterminals S and substrate 202. Each second terminal has connected to itone electrode closer to the substrate and one electrode further from thesubstrate. In the example shown, one second terminal has electrode 212closer to the substrate and electrode 254 further from the substrateconnected to it and the other second terminal has electrode 252 closerto the substrate and electrode 214 further from the substrate connectedto it. Thus, the embodiment of thin-film acoustically-coupledtransformer 200 shown in FIG. 4D is electrically balanced and, as aresult, has a common-mode rejection ratio sufficiently high for manymore applications than the thin-film acoustically-coupled transformer100 described above with reference to FIGS. 1A-1C.

The embodiment of thin-film acoustically-coupled transformer 200 shownin FIG. 4D is a step-down transformer when first terminals F serve asprimary terminals and second terminals S serve as secondary terminals.The signal level output at the secondary terminals is one-half that ofthe input electrical signal applied to the primary terminals. Also, in atypical embodiment in which all of the FBARs 210, 220, 250 and 260 havea similar characteristic impedance, the impedance seen at the primaryterminals is that of two FBARs in series, i.e., twice the typicalcharacteristic impedance of a single FBAR, whereas the impedance seen atthe secondary terminals is that of two FBARs in parallel, i.e., one-halfof the typical characteristic impedance of a single FBAR. Thus, theembodiment of thin-film acoustically-coupled transformer 200 illustratedin FIG. 4D has a 4:1 primary-to-secondary impedance ratio.

The embodiment of thin-film acoustically-coupled transformer 200 shownin FIG. 4D is a step-up transformer when first terminals F serve assecondary terminals and second terminals S serve as primary terminals.In this case, the signal level output at the secondary terminals istwice that of the input electrical signal applied to the primaryterminals, and the primary-to-secondary impedance ratio is 1:4.

In applications in which a low common mode rejection ratio isunimportant, electrical circuits interconnecting the FBARs can bedifferent from those just described. FIG. 4E shows an embodiment of anacoustically-coupled transformer with a 1:1 impedance transformationratio in which an electrical circuit 341 connects FBAR 220 of SBAR 206and FBAR 260 of SBAR 208 in parallel and to first terminals F, and anelectrical circuit 342 electrically connects FBAR 210 of SBAR 206 andFBAR 250 of SBAR 208 in parallel and to second terminals S.

FIG. 4F shows an embodiment of an acoustically-coupled transformer witha 1:1 impedance transformation ratio in which an electrical circuit 343connects FBAR 220 of SBAR 206 and FBAR 260 of SBAR 208 in anti-seriesbetween first terminals F, and an electrical circuit 344 connects FBAR210 of SBAR 206 and FBAR 250 of SBAR 208 in anti-series between secondterminals S.

FIG. 4G shows an embodiment of an acoustically-coupled transformer inwhich an electrical circuit 345 electrically connects FBAR 220 of SBAR206 and FBAR 260 of SBAR 208 in parallel and to first terminals F, andan electrical circuit 346 electrically connects FBAR 210 of SBAR 206 andFBAR 250 of SBAR 208 in anti-series between second terminals S. Thisembodiment has a 1:4 impedance transformation ratio when first terminalsF serve as primary terminals and second terminals S serve as secondaryterminals, or a 4:1 impedance transformation ratio when second terminalsS serve as the primary terminals and first terminals F serve as thesecondary terminals.

FIG. 4H shows an embodiment of an acoustically-coupled transformer inwhich electrical circuit 347 electrically connects FBAR 220 of SBAR 206and FBAR 260 of SBAR 208 in anti-series between first terminals F, andan electrical circuit 348 electrically connects FBAR 210 of SBAR 206 andFBAR 250 of SBAR 208 in parallel and to second terminals S. Thisembodiment has a 4:1 impedance transformation ratio when first terminalsF serve as primary terminals and second terminals S serve as secondaryterminals, or a 1:4 impedance transformation ratio when second terminalsS serve as the primary terminals and first terminals F serve as thesecondary terminals.

The electrical configuration of the embodiment of the thin-filmacoustically-coupled transformer 200 shown in FIGS. 3A-3C is similar tothat shown in FIG. 4C. A bonding pad 282 and a bonding pad 284constitute the first terminals of thin-film acoustically-coupledtransformer 200. An interconnection pad 236, an electrical trace 237extending from electrode 222 to interconnection pad 236 (FIG. 5G), aninterconnection pad 278 in electrical contact with interconnection pad236 and an electrical trace 279 extending from electrode 264 tointerconnection pad 278 constitute the part of electrical circuit 245(FIG. 4C) that electrically connects electrode 222 of FBAR 220 toelectrode 264 of FBAR 260. An interconnection pad 238, an electricaltrace 239 extending from electrode 224 to interconnection pad 238, aninterconnection pad 276 in electrical contact with interconnection pad238 and an electrical trace 277 extending from electrode 262 tointerconnection pad 276 (FIG. 5G) constitute the part of electricalcircuit 245 (FIG. 4C) that electrically connects electrode 224 of FBAR220 to electrode 262 of FBAR 260. An electrical trace 283 that extendsbetween electrode 222 and bonding pad 282 and an electrical trace 285that extends between electrode 264 and bonding pad 284 (FIG. 5G)constitute the part of electrical circuit 245 that connects FBARs 220and 260 connected in anti-parallel to the first terminals provided bybonding pads 282 and 284.

In an alternative embodiment, bonding pads 282 and 284 and traces 283and 285 are omitted and interconnection pads 238 and 278 are configuredas bonding pads and provide the first terminals of thin-filmacoustically-coupled transformer 200.

Bonding pad 232 and bonding pad 272 constitute the second terminals ofthin-film acoustically-coupled transformer 200. An electrical trace 235that extends between electrode 214 and electrode 254 (FIG. 5E)constitutes the part of electrical circuit 246 (FIG. 4C) that connectsFBAR 210 and FBAR 250 in series. An electrical trace 233 that extendsbetween electrode 212 and bonding pad 232 and an electrical trace 273that extends between electrode 252 and bonding pad 272 (FIG. 5C)constitutes the part of electrical circuit 246 that connects FBAR 210and FBAR 250 to the second terminals provided by bonding pads 232 and272.

In thin-film acoustically-coupled transformer 200, acoustic decoupler230 is located between FBARs 210 and 220, specifically, betweenelectrodes 214 and 222. Acoustic decoupler 230 controls the coupling ofacoustic energy between FBARs 210 and 220. Additionally, acousticdecoupler 270 is located between FBARs 250 and 260, specifically,between electrodes 254 and 262. Acoustic decoupler 270 controls thecoupling of acoustic energy between FBARs 250 and 260. Acousticdecoupler 230 couples less acoustic energy between the FBARs 210 and 220than would be coupled if the FBARs were in direct contact with oneanother. Acoustic decoupler 270 couples less acoustic energy between theFBARs 250 and 260 than would be coupled if the FBARs were in directcontact with one another. The coupling of acoustic energy defined byacoustic decouplers 230 and 270 determines the pass bandwidth ofthin-film acoustically-coupled transformer 200.

In the embodiment shown in FIGS. 3A-3C, acoustic decouplers 230 and 270are respective parts of a layer 231 of acoustic decoupling material.Important properties of the acoustic decoupling material of layer 231are an acoustic impedance less than that of FBARs 210, 220, 250 and 260,a nominal thickness that is an odd integral multiple of one quarter ofthe wavelength in the acoustic decoupling material of an acoustic wavehaving a frequency equal to the center frequency of the pass band of thetransformer 200, and a high electrical resistivity and low dielectricpermittivity to provide electrical isolation between the primary andsecondary of the transformer. The materials and other properties oflayer 231 are similar to those described above with reference to FIGS.1A-1D and FIG. 2. Therefore, layer 231 that provides acoustic decouplers230 and 270 will not be further described here. In another embodiment(not shown), acoustic decouplers 230 and 270 each include a Braggstructure similar to Bragg structure 161 described above with referenceto FIG. 1E. Acoustic decouplers 230 and 270 may alternatively share acommon Bragg structure in a manner similar to the way in which theembodiments of acoustic couplers 230 and 270 shown in FIGS. 3A-3C sharea common layer 231.

SBAR 206 and SBARs 208 are located adjacent one another suspended over acavity 204 defined in a substrate 202. Suspending the SBARs over acavity allows the stacked FBARs in each SBAR to resonate mechanically.Other suspension schemes that allow the stacked FBARs to resonatemechanically are possible. For example, the SBARs can be located over amismatched acoustic Bragg reflector (not shown) formed in or onsubstrate 202, as disclosed by the above-mentioned U.S. Pat. No.6,107,721 of Lakin.

Thousands of thin-film acoustically-coupled transformers similar tothin-film acoustically-coupled transformer 200 are fabricated at a timeby wafer-scale fabrication. Such wafer-scale fabrication makes thethin-film acoustically-coupled transformers inexpensive to fabricate. Anexemplary fabrication method will be described next with reference tothe plan views of FIGS. 5A-5J and the cross-sectional views of FIGS.5K-5T. As noted above, the fabrication method can also be used to makethe thin-film acoustically-coupled transformer 100 described above withreference to FIGS. 1A-1C.

A wafer of single-crystal silicon is provided. A portion of the waferconstitutes, for each transformer being fabricated, a substratecorresponding to the substrate 202 of transformer 200. FIGS. 5A-5J andFIGS. 5K-5T illustrate and the following description describes thefabrication of transformer 200 in and on a portion of the wafer. Astransformer 200 is fabricated, the remaining transformers on the waferare similarly fabricated.

The portion of the wafer that constitutes substrate 202 of transformer200 is selectively wet etched to form cavity 204, as shown in FIGS. 5Aand 5K.

A layer of fill material (not shown) is deposited on the surface of thewafer with a thickness sufficient to fill the cavities. The surface ofthe wafer is then planarized to leave the cavity filled with the fillmaterial. FIGS. 5B and 5L show cavity 204 in substrate 202 filled withfill material 205.

In an embodiment, the fill material was phosphosilicate glass (PSG) andwas deposited using conventional low-pressure chemical vapor deposition(LPCVD). The fill material may alternatively be deposited by sputtering,or by spin coating.

A layer of metal is deposited on the surface of the wafer and the fillmaterial. The metal is patterned to define electrode 212, bonding pad232, an electrical trace 233 extending between electrode 212 and bondingpad 232, electrode 252, bonding pad 272 and an electrical trace 273extending between electrode 212 and bonding pad 272, as shown in FIGS.5C and 5M. Electrode 212 and electrode 252 typically have an irregularshape in a plane parallel to the major surface of the wafer. Anirregular shape minimizes lateral modes in FBAR 210 and FBAR 250 (FIG.3A) of which the electrodes form part, as described in U.S. Pat. No.6,215,375 of Larson III et al., the disclosure of which is incorporatedinto this disclosure by reference. Electrode 212 and electrode 252 arelocated to expose part of the surface of fill material 205 so that thefill material can later be removed by etching, as will be describedbelow.

The metal layers in which electrodes 212, 214, 222, 224, 252, 254, 262and 264 are defined are patterned such that, in respective planesparallel to the major surface of the wafer, electrodes 212 and 214 ofFBAR 210 have the same shape, size, orientation and position, electrodes222 and 224 of FBAR 220 have the same shape, size, orientation andposition, electrodes 252 and 254 of FBAR 250 have the same shape, size,orientation and position and electrodes 262 and 264 of FBAR 260 have thesame shape, size, orientation and position. Typically, electrodes 214and 222 additionally have the same shape, size, orientation and positionand electrodes 254 and 262 additionally have the same shape, size,orientation and position.

In an embodiment, the metal deposited to form electrode 212, bonding pad232, trace 233, electrode 252, bonding pad 272 and trace 273 wasmolybdenum. The molybdenum was deposited with a thickness of about 440nm by sputtering, and was patterned by dry etching to define pentagonalelectrodes each with an area of about 26,000 square μm. Other refractorymetals such as tungsten, niobium and titanium may alternatively be usedas the material of electrodes 212 and 252, bonding pads 232 and 272 andtraces 233 and 273. The electrodes, bonding pads and traces mayalternatively comprise layers of more than one material.

A layer of piezoelectric material is deposited and is patterned todefine a piezoelectric layer 217 that provides piezoelectric layer 216of FBAR 210 and piezoelectric layer 256 of FBAR 250, as shown in FIGS.5D and 5N. Piezoelectric layer 217 is patterned to expose part of thesurface of fill material 205 and bonding pads 232 and 272. Piezoelectriclayer 217 is additionally patterned to define windows 219 that provideaccess to additional parts of the surface of the fill material.

In an embodiment, the piezoelectric material deposited to formpiezoelectric layer 217 was aluminum nitride and was deposited with athickness of about 780 nm by sputtering. The piezoelectric material waspatterned by wet etching in potassium hydroxide or by chlorine-based dryetching. Alternative materials for piezoelectric layer 217 include zincoxide and lead zirconium titanate.

A layer of metal is deposited and is patterned to define electrode 214,electrode 254 and electrical trace 235 extending between electrode 214and electrode 254, as shown in FIGS. 5E and 5O.

In an embodiment, the metal deposited to form electrode 214, electrode254 and trace 235 was molybdenum. The molybdenum was deposited with athickness of about 440 nm by sputtering, and was patterned by dryetching. Other refractory metals may alternatively be used as thematerial of electrodes 214 and 254 and trace 235. The electrodes andtrace may alternatively comprise layers of more than one material.

A layer of acoustic decoupling material is then deposited and ispatterned to define an acoustic decoupling layer 231 that providesacoustic decoupler 230 and acoustic decoupler 270, as shown in FIGS. 5Fand 5P. Acoustic decoupling layer 231 is shaped to cover at leastelectrode 214 and electrode 254, and is additionally shaped to exposepart of the surface of fill material 205 and bonding pads 232 and 272.Acoustic decoupling layer 231 is additionally patterned to definewindows 219 that provide access to additional parts of the surface ofthe fill material.

In an embodiment, the acoustic decoupling material was polyimide with athickness of about 750 nm, i.e., three quarters of the center frequencywavelength in the polyimide. The polyimide was deposited to formacoustic decoupling layer 231 by spin coating, and was patterned byphotolithography. Polyimide is photosensitive so that no photoresist isneeded. As noted above, other plastic materials can be used as theacoustic decoupling material. The acoustic decoupling material can bedeposited by methods other than spin coating.

In an embodiment in which the material of the acoustic decoupling layer231 was polyimide, after deposition and patterning of the polyimide, thewafer was baked at about 300° C. before further processing wasperformed. The bake evaporates volatile constituents of the polyimideand prevents the evaporation of such volatile constituents duringsubsequent processing from causing separation of subsequently-depositedlayers.

A layer of metal is deposited and is patterned to define electrode 222,interconnection pad 236, electrical trace 237 extending from electrode222 to interconnection pad 236, bonding pad 282 and electrical trace 283extending from electrode 222 to bonding pad 282, as shown in FIGS. 5Gand 5Q. The patterning also defines in the layer of metal electrode 262,interconnection pad 276 and electrical trace 277 extending fromelectrode 262 to interconnection pad 276, also as shown in FIGS. 5G and5Q.

In an embodiment, the metal deposited to form electrodes 222 and 262,bonding pad 282, interconnection pads 236 and 276 and electrical traces237, 277 and 283 was molybdenum. The molybdenum was deposited with athickness of about 440 nm by sputtering, and was patterned by dryetching. Other refractory metals may alternatively be used as thematerial of electrodes 222 and 262, pads 236, 276 and 282 and electricaltraces 237, 277 and 283. The electrodes, bonding pads and traces mayalternatively comprise layers of more than one material.

A layer of piezoelectric material is deposited and is patterned todefine piezoelectric layer 227 that provides piezoelectric layer 226 ofFBAR 220 and piezoelectric layer 266 of FBAR 260. Piezoelectric layer227 is shaped to expose pads 232, 236, 272, 276 and 282 and to exposepart of the surface of fill material 205 as shown in FIGS. 5H and 5R.Piezoelectric layer 227 is additionally patterned to define the windows219 that provide access to additional parts of the surface of the fillmaterial.

In an embodiment, the piezoelectric material deposited to formpiezoelectric layer 227 was aluminum nitride and was deposited with athickness of about 780 nm by sputtering. The piezoelectric material waspatterned by wet etching in potassium hydroxide or by chlorine-based dryetching. Alternative materials for piezoelectric layer 227 include zincoxide and lead zirconium titanate.

A layer of metal is deposited and is patterned to define electrode 224,interconnection pad 238 and electrical trace 239 extending fromelectrode 224 to interconnection pad 238, as shown in FIGS. 51 and 5S.Interconnection pad 238 is located in electrical contact withinterconnection pad 276 to provide the part of electrical circuit 245(FIG. 4C) that connects electrodes 224 and 262. The patterning alsodefines in the layer of metal electrode 264, interconnection pad 278,electrical trace 279 extending from electrode 264 to interconnection pad278, bonding pad 284 and electrical trace 285 extending from electrode264 to bonding pad 284, also as shown in FIGS. 5I and 5S.Interconnection pad 278 is located in electrical contact withinterconnection pad 236 to provide the part of electrical circuit 245(FIG. 4C) that connects electrodes 222 and 264. As noted above, bondingpads 282 and 284 and electrical traces 283 and 285 may be omitted ifreliable electrical connections can be made to stacked interconnectionpads 236 and 278 and to stacked interconnection pads 276 and 238.

In an embodiment, the metal deposited to form electrodes 224 and 264,pads 238, 278 and 284 and electrical traces 237, 279 and 285 wasmolybdenum. The molybdenum was deposited with a thickness of about 440nm by sputtering, and was patterned by dry etching. Other refractorymetals such may alternatively be used as the material of electrodes 224and 264, pads 238, 278 and 284 and electrical traces 237, 279 and 285.The electrodes, pads and traces may alternatively comprise layers ofmore than one material.

The wafer is then isotropically wet etched to remove fill material 205from cavity 204. As noted above, portions of the surface of fillmaterial 205 remain exposed through, for example, windows 219. The etchprocess leaves thin-film acoustically-coupled transformer 200 suspendedover cavity 204, as shown in FIGS. 5J and 5T.

In an embodiment, the etchant used to remove fill material 205 wasdilute hydrofluoric acid.

A gold protective layer is deposited on the exposed surfaces of pads232, 238, 272, 278, 282 and 284.

The wafer is then divided into individual transformers, includingtransformer 200. Each transformer is then mounted in a package andelectrical connections are made between bonding pads 232, 272, 282 and284 of the transformer and pads that are part of the package.

A process similar to that described may be used to fabricate embodimentsof thin-film acoustically-coupled transformer 200 in which the FBARs areelectrically connected as shown in FIGS. 4B-4H.

In use, bonding pad 282 electrically connected to electrodes 222 and 264and bonding pad 284 electrically connected to electrodes 224 and 262provide the first terminals of the transformer 200, and bonding pad 232electrically connected to electrode 212 and bonding pad 272 electricallyconnected to electrode 252 provide the second terminals of transformer200. In one embodiment, the first terminals provide the primaryterminals and the second terminals provide the secondary terminals ofthin-film acoustically-coupled transformer 200. In another embodiment,the first terminals provide the secondary terminals and the secondterminals provide the primary terminals of thin-filmacoustically-coupled transformer 200.

An embodiment of thin-film acoustically-coupled transformer 200 in whichacoustic decoupler 130 incorporates a Bragg structure similar to thatdescribed above with reference to FIG. 1E is made by a process similarto that described above. The process differs as follows:

After a layer 217 of piezoelectric material is deposited and patterned(FIGS. 5D and 5N), a layer of metal is deposited and is patterned todefine a high acoustic impedance Bragg element incorporating electrodes214 and 254 and additionally to define electrical trace 235 extendingbetween the electrodes, in a manner similar to that shown in FIGS. 5Eand 50. The high acoustic impedance Bragg element is similar to highacoustic impedance Bragg element 165 shown in FIG. 1E. The layer ofmetal is deposited with a nominal thickness equal to an odd, integralmultiple of one quarter of the wavelength in the metal of an acousticwave having a frequency equal to the center frequency of the pass bandof transformer 200.

In an embodiment, the metal deposited to form the high acousticimpedance Bragg element incorporating electrodes 214 and 254 ismolybdenum. The molybdenum is deposited with a thickness of about 820 nm(one-quarter wavelength in Mo) by sputtering, and is patterned by dryetching. Other refractory metals may alternatively be used as thematerial of the high acoustic impedance Bragg element incorporatingelectrodes 214 and 254. The high acoustic impedance Bragg element mayalternatively comprise layers of more than one metal.

A layer of low acoustic impedance material is then deposited and ispatterned to define a low acoustic impedance Bragg element in a mannersimilar to that shown in FIGS. 5F and 5P. The layer of low acousticimpedance material is deposited with a nominal thickness equal to anodd, integral multiple of one quarter of the wavelength in the lowacoustic impedance material of an acoustic wave having a frequency equalto the center frequency of the pass band of transformer 200. The lowacoustic impedance Bragg element is shaped to cover at least the highacoustic impedance Bragg element, and is additionally shaped to exposepart of the surface of fill material 205 and bonding pads 232 and 272.The layer of low acoustic impedance material is additionally patternedto define windows that provide access to additional parts of the surfaceof the fill material.

In an embodiment, the low acoustic impedance material is SiO₂ with athickness of about 790 nm. The SiO₂ is deposited by sputtering, and ispatterned by etching. Other low acoustic impedance material that can beused as the material of low acoustic impedance Bragg element includephosphosilicate glass (PSG), titanium dioxide and magnesium fluoride.The low acoustic impedance material can alternatively be deposited bymethods other than sputtering.

A layer of metal is deposited and is patterned to define a high acousticimpedance Bragg element incorporating electrodes 222 and 262. The layerof metal is additionally patterned to define an interconnection pad 236,an electrical trace 237 extending from electrode 222 to interconnectionpad 236, a bonding pad 282, an electrical trace 283 extending fromelectrode 222 to bonding pad 282, an interconnection pad 276 and anelectrical trace 277 extending from electrode 262 to interconnection pad276 in a manner similar to that shown in FIGS. 7G and 7Q. The layer ofmetal is deposited with a nominal thickness equal to an odd, integralmultiple of one quarter of the wavelength in the metal of an acousticwave having a frequency equal to the center frequency of the pass bandof transformer 200.

In an embodiment, the metal deposited to form a high acoustic impedanceBragg element incorporating electrodes 222 and 262 is molybdenum. Themolybdenum is deposited with a thickness of about 820 nm (one-quarterwavelength in Mo) by sputtering, and is patterned by dry etching. Otherrefractory metals may alternatively be used as the material of the highacoustic impedance Bragg element incorporating electrodes 222 and 262and its associated pads and electrical traces. The high acousticimpedance Bragg element, pads and electrical traces may alternativelycomprise layers of more than one material.

A layer of piezoelectric material is then deposited and is patterned todefine piezoelectric layer 227, as described above, and the processcontinues as described above to complete fabrication of transformer 200.

This disclosure describes the invention in detail using illustrativeembodiments. However, the invention defined by the appended claims isnot limited to the precise embodiments described.

1. An acoustically-coupled transformer, comprising: a stacked bulkacoustic resonator (SBAR), comprising: a stacked pair of film bulkacoustic resonators (FBARs), each of the FBARs comprising opposed planarelectrodes and a layer of piezoelectric material between the electrodes,and an acoustic decoupler between the FBARs; first terminalselectrically connected to the electrodes of one of the FBARs; and secondterminals electrically connected to the electrodes of the other of theFBARs.
 2. The transformer of claim 1, in which the acoustic decouplercomprises a layer of acoustic decoupling material.
 3. The transformer ofclaim 2, in which: the piezoelectric material has an acoustic impedance;and the acoustic decoupling material has an acoustic impedance less thanthe acoustic impedance of the piezoelectric material.
 4. The transformerof claim 3, in which the acoustic decoupling material has an acousticimpedance intermediate between the acoustic impedance of thepiezoelectric material and the acoustic impedance of air.
 5. Thetransformer of claim 2, in which the acoustic decoupling material has anacoustic impedance in the range from about 2 Mrayl to about 16 Mrayl. 6.The transformer of claim 2, in which the acoustic decoupling materialcomprises plastic.
 7. The transformer of claim 2, in which the acousticdecoupling material comprises polyimide.
 8. The transformer of claim 2,in which the acoustic decoupling material comprises poly(para-xylylene).9. The transformer of claim 2, in which: the transformer has a pass bandcharacterized by a center frequency; and the layer of acousticdecoupling material has a nominal thickness equal to an odd integralmultiple of one quarter of the wavelength in the acoustic decouplingmaterial of an acoustic wave having a frequency equal to the centerfrequency.
 10. The transformer of claim 1, in which the acousticdecoupler comprises a Bragg structure.
 11. The transformer of claim 10,in which the Bragg structure comprises one or more low acousticimpedance Bragg elements interleaved with high acoustic impedance Braggelements.
 12. The transformer of claim 11, in which two of the highacoustic impedance Bragg elements additionally serve as one of theelectrodes of each of the FBARs.
 13. The transformer of claim 1, inwhich: the SBAR is a first SBAR; and the transformer additionallycomprises: a second SBAR, comprising: a stacked pair of FBARs, each FBARcomprising opposed planar electrodes and a layer of piezoelectricmaterial between the electrodes; and an acoustic decoupler between theFBARs, a first electrical circuit connecting one of the FBARs of thefirst SBAR to one of the FBARs of the second SBAR and to the firstterminals, and a second electrical circuit connecting the other of theFBARs of the first SBAR to the other of the FBARs of the second SBAR andto the second terminals.
 14. The transformer of claim 13, in which theacoustic decoupler of each of the SBARs comprises a layer of acousticdecoupling material.
 15. The transformer of claim 14, in which: thepiezoelectric material of each of the SBARs has an acoustic impedance;and the acoustic decoupling material has an acoustic impedance less thanthe acoustic impedance of the piezoelectric material.
 16. Thetransformer of claim 14, in which the acoustic decoupling material hasan acoustic impedance in the range from about 2 Mrayl to about 16 Mrayl.17. The transformer of claim 14, in which: the transformer has a passband characterized by a center frequency; and the acoustic decouplingmaterial has a nominal thickness equal to an odd integral multiple ofone quarter of the wavelength in the acoustic decoupling material of anacoustic wave having a frequency equal to the center frequency.
 18. Thetransformer of claim 13, in which the acoustic decoupler of each of theSBARs comprises a Bragg stack.
 19. The transformer of claim 18, in whichthe Bragg structure comprises one or more low acoustic impedance Braggelements interleaved with high acoustic impedance Bragg elements. 20.The transformer of claim 19, in which two of the high acoustic impedanceBragg elements additionally serve as one of the electrodes of each ofthe FBARs.
 21. An acoustically-coupled transformer, comprising: a firststacked bulk acoustic resonator and a second stacked bulk acousticresonator (SBAR), each SBAR comprising: a stacked pair of film bulkacoustic resonators (FBARs), each of the FBARs comprising opposed planarelectrodes and a layer of piezoelectric material between the electrodes,and an acoustic decoupler between the FBARs; a first electrical circuitconnecting one of the FBARs of the first SBAR to one of the FBARs of thesecond SBAR; and a second electrical circuit connecting the other of theFBARs of the first SBAR to the other of the FBARs of the second SBAR.22. The transformer of claim 21, in which: the first electrical circuitconnects the one of the FBARs of the first SBAR in one of (a) series and(b) anti-parallel with the one of the FBARs of the second SBAR; and thesecond electrical circuit connects the other of the FBARs of the firstSBAR in one of (c) series and (d) anti-parallel with the other of theFBARs of the second SBAR.
 23. The transformer of claim 22, in which: thetransformer additionally comprises first terminals and second terminals;the first electrical circuit additionally connects the ones of the FBARsto the first terminals; and the second electrical circuit additionallyconnects the others of the FBARs to the second terminals.
 24. Thetransformer of claim 23, in which the first terminals constitute primaryterminals and the second terminals constitute secondary terminals. 25.The transformer of claim 22, in which: the transformer additionallycomprises a substrate arranged support the SBARs; in the one of theFBARs of each the SBARs, one of the electrodes thereof is a firstelectrode, the other of the electrodes thereof is a second electrode,and the first electrode is closer to the substrate than the secondelectrode; and the first electrical circuit comprises: an electricalconnection between the first electrode of the first SBAR and the secondelectrode of the second SBAR, and an electrical connection between thesecond electrode of the first SBAR and the first electrode of the secondSBAR.
 26. The transformer of claim 21, in which: the first electricalcircuit connects the one of the FBARs of the first SBAR in one of (a)parallel and (b) anti-series with the one of the FBARs of the secondSBAR; and the second electrical circuit connects the other of the FBARsof the first SBAR in one of (c) parallel and (d) anti-series with theother of the FBARs of the second SBAR.
 27. The transformer of claim 26,in which: the transformer additionally comprises first terminals andsecond terminals; the first electrical circuit additionally connects theones of the FBARs to the first terminals; and the second electricalcircuit additionally connects the others of the FBARs to the secondterminals.
 28. The transformer of claim 26, in which the first terminalsconstitute primary terminals and the second terminals constitutesecondary terminals.
 29. The transformer of claim 21, in which theacoustic decouplers each comprise a layer of acoustic decouplingmaterial.
 30. The transformer of claim 21, in which the acousticdecouplers each comprise a Bragg structure.